This invention relates to a device for detecting the unbalance between two fractions of the return traction current flowing in the two rails of a railroad track, in the presence of a signalling current in the respective track circuit.
It is known that a railroad electric locomotive is fed with current from an external generator (usually from a feeding substation) through an overhead wire, and said current is returned through the pair of rails of the railroad track. Usually, the return traction current is divided into two roughly equal fractions between the two rails. In case of electrical asymmetry between the two rails, however, the corresponding fractions of the return traction current are different from each other. The difference between the two fractions of the return traction current in the two rails of a railroad track is called "unbalance" and can cause disturbances in the normal signalling made through the track circuits, particularly when the traction current, in addition to the continuous component, contains spectral components with frequencies, for example, between 30 and 70 Hz and the signalling current in the track circuit is also an alternating current, e.g. a 50 Hz current which is amplitude-modulated with signals having, for example, frequencies of 1.25-2-3-4.5 Hz.
To detect the value of unbalance .DELTA.i between the two fractions of the return traction current in the two track rails, the invention is based on the recognition that--in the presence of a signalling current in the track--said unbalance .DELTA.i is proportional to the sum (i.sub.1 +i.sub.2) of the currents i.sub.1 and i.sub.2 actually present in the rails and formed each by a component of the traction current and by a component of the track signalling current, said proportionality corresponding to a coefficient which is equal to the ratio between the mean values of the difference (i.sub.1 -i.sub.2) and of the sum (i.sub.1 +i.sub.2) of said currents actually present in the two rails, said difference and said sum being multiplied each by a given function M(t).
The above is expressed by the formula: ##EQU2## wherein: .DELTA.i=unbalance between the two fractions of the return traction current in the two track rails;
i.sub.1, i.sub.2 =currents actually present in the two track rails and formed each by the track signalling current and by a fraction of the total current of traction.
The above formula is not valid if EQU Fo[M.vertline.i.sub.1 +i.sub.2 .vertline.]=0
In this instance (i.e., the denominator in formula 1 is zero), the following two conditions occur:
(1) If the mean value of the sum of actual currents i.sub.1 and i.sub.2 flowing in the two rails is higher than the mean value of the difference between these currents, namely if Fo[.vertline.i.sub.1 +i.sub.2 .vertline.]&gt;Fo[.vertline.i.sub.1 -i.sub.2 .vertline.], then: EQU .DELTA.i=i.sub.1 -i.sub.2 ( 2)
(2) However, if the mean value of the sum of actual currents i.sub.1 and i.sub.2 flowing in the two rails is lower than the mean value of the difference between said currents, namely if Fo[.vertline.i.sub.1 +i.sub.2 .vertline.]&lt;Fo[.vertline.i.sub.1 -i.sub.2 .vertline.], then: EQU .DELTA.i=i.sub.1 +i.sub.2 ( 3)
Finally, when the two currents i.sub.1 and i.sub.2 actually present in the two rails are such as to prevent the recognition of the signalling current therein, due to the unbalance .DELTA.i of the two respective fractions of the return traction current, the formula (2) should be valid.
In conclusion, the amount of unbalance .DELTA.i between the two fractions of the return traction current in the two track rails may be determined, depending upon the circumstances, by one of the three formulae (1), (2), or (3).